Abstract:

Oil and gas reservoirs discovered in the ultra-deep carbonates of a burial depth larger than 7 000 m in Shunbei area of Tarim Basin have gained attention from the industry.A comprehensive study was then carried out on the tectonic evolution,reservoir characteristics and oil and gas accumulation patterns in the area.The results show that the reservoirs are of fault-karst type,featuring pervasive distribution but uneven enrichment of oil in the whole fault zone.With caverns,fractures and along-fracture dissolved pores and vugs formed by multi-stage strike-slip faulting as well as dissolution and alteration,the Middle-Lower Ordovician carbonates were wonderful reservoirs with favorable properties.Tight carbonates at the periphery of the faulting zone served well as lateral barriers for the formation of lithologic traps.The overlaying massive mudstone in Queerqueke Formation was the regional seal,which caped the hydrocarbons being sourced from the Lower Cambrian and migrating up along the strike-slip faults. The accumulations were mostly formed at late stage.The faults acted both as hydrocarbon migration pathways and accumulation spots.The discovery of the reservoirs demonstrated a new hydrocarbon reservoiring pattern in the ultra-deep sequences.About 18 strike-slip faulting zones were identified in the area with a resource volume as high as 17×10⁸ t oil equivalent,indicating a bright exploration and development future in the area.

Key words: ultra-deep layer, carbonate, fault-karst, oil/gas accumulation, Shunbei area, Tarim Basin